(1) Field of the Invention
The invention relates generally to electronic circuits for telecommunications and to methods used therewith and more particularly, to a circuit for transmission of sound signals and to a method for speech transmission with noise suppression. The invention also concerns an apparatus for implementing the method and use thereof.
(2) Description of the Prior Art
In telecommunications and recording techniques of sound signals a major problem is the degradation of the transmitted or recorded sound signals by ambient noise. When it comes to speech transmission or recording the intelligibility of the transmitted or recorded speech signal in the presence of audible noise is most important. This is especially very obvious and significant in the case where car drivers are communicating on telephone during ride with the aid of hands-free phone equipment. In order to generally suppress or reduce audible ambient noise of such sound signals a multitude of techniques and methods has been specified in the past.
The main problem hereby is due to the fact, that in most cases the unwanted noise signal and the wanted sound signal are most likely to appear within the same frequency range. Such they have to be discriminated by other characteristics than their frequency range. Albeit filtering techniques in the frequency domain have been vastly used in prior art, yet with unsatisfactory results. Other discrimination characteristics, both in the frequency and in the time domain have been under scrutiny in many different prior art approaches and have proved to deliver more satisfying results. Modern digital integrated signal processing circuits either built up with discrete computational units or in the form of monolithic digital signal processors allow for an extensive use of advanced calculation algorithms such as the Fast/Discrete Fourier Transformation (FFT/DFT) or Correlation Analysis (CA) methods. The computational demands hereby are however very high and are often not suitable for real-time applications. In case where real-time requirements have to be met, practical realizations lead to very costly solutions.
FIG. 1A prior art depicts the normally used method for the processing in such digital integrated signal processing circuits, whereby in block 15 the Fast Fourier Transformation (FFT) processing is taking place, namely for all the M samples of an incoming noisy signal x(n) during one sampling period, giving M FFT values X(n,k), whereby n may be called a ‘discrete time variable’ for x(n) and k named as a ‘normalized frequency number or index’ in case of X(n,k). These M results X(n,k) 35 are then carried altogether in parallel into the Noise Reduction Processing Unit 55 for their further processing to achieve the desired “noise free” resulting signal s(n), whereby the calculations for all frequency numbers are done all at once, which is very time consuming and thus causing considerable delay for the processing of a whole data set due to the many calculations needed. As can also be seen substantial computing power in blocks 15 and 55 is needed for all these necessary calculations.
It is therefore a challenge for the designer of such methods and circuits to achieve a high-quality and low-cost solution. Several prior art inventions referring to such solutions describe related technologies, methods and circuits.
U.S. Pat. No. 6,208,951 (to Kumar et al.) describes a method and an apparatus for the identification and/or separation of complex composite signals into its deterministic and noisy components with a given process for the identification and/or separation of composite signal into its deterministic and noisy components wherein the process uses recursive wavelet transformations to separate the deterministic and noisy components of signals and uses the difference in the properties with regard to degree of correlation and dimensionality of these constituent components as a basis for separation, the said process of identification and/or separation has application in a variety of situations where digitized data is made available via an apparatus which converts the monitored signals.
U.S. Pat. No. 6,502,067 (to Hegger et al.) discloses a method and apparatus for processing noisy sound signals including a method for processing a sound signal y in which redundancy, consisting mainly of almost repetitions of signal profiles, is detected and correlations between the signal profiles are determined within segments of the sound signal. Correlated signal components are allocated to a power component and uncorrelated signal components to a noise component of the sound signal. The correlations between the signal profiles are determined by methods of nonlinear noise reduction in deterministic systems in reconstructed vector spaces based on the time domain.
Canadian Patent CA 02319995 (to Ruwisch) discloses a method and apparatus for suppressing audible noise in speech transmission by means of a multi-layer self-organizing fed-back neural network. This method involves using a multi-layer self-organising neural network with feedback. A minima detection layer, a reaction layer, a diffusion layer and an integration layer define a filter function (F(f,T)) for noise filtering. The filter function is used to convert a spectrum B(f, T) free of noise, into a noise-free speech signal (y(t)) by inverse Fourier transformation. The signal delay caused by processing the signal is so short that the filter can operate in real-time for telecommunication. All neurons are supplied with an externally set parameter K, the size of which defines the degree of noise suppression of the whole filter. An Independent claim is included for an apparatus for noise suppression during speech transmission.
The Ph.D. thesis of Hyoung-Gook Kim, “Background Noise Reduction Based on Diffusive Gain Factors and 1.2 kbit/s Low Bit Rate Speech Coding Using Spectral Vector Quantization of Differential Features, Technische Universität Berlin, Fachbereich Elektrotechnik und Informatik, Berlin 2002, D83” describes a novel method which uses a background noise reduction with the help of a minimum detection stage, a stage for the estimation of the noise and a computation stage based on Diffusive Gain Factors (DGF). The circuit developed for this method has however a rather high demand for processing power.
Although these papers describe methods close to the field of the invention they differ in essential features from the method and especially the circuit introduced here.